1. Technical Field
This invention relates to the technology of electrically isolating electrically functioning elements, and more particularly to an improved process for depositing electrically insulating thermoplastic coatings onto an unroughened or roughened substrate by flame spraying or equivalent spraying that cures the coating immediately.
2. Discussion of the Prior Art
Ceramic coatings are often used to electrically isolate elements in an assembly, such as isolating an alternator diode plate from high current in an adjacent member in an automotive alternator. In an automotive alternator, a rectifier employs negative and positive diode plates to convert A.C. current to D.C. These plates need to be electrically isolated but allow for thermal conductivity therebetween. Aluminum oxide is a typical ceramic candidate that is used for these coatings; it is usually applied in a four step process, comprising; (i) roughening a plate surface by grit blasting to receive the coating, (ii) masking parts of the surface (usually with consumable hardened steel) that do not receive the coating, (iii) using an argon/hydrogen plasma spraying process to melt and deposit a powder supply of Al.sub.2 O.sub.3, and (iv) sealing the porosity of the ceramic coating with an ultraviolet curable plastic overcoat. This process is costly and the ceramic is difficult to machine once deposited. It is also complex, requiring separate processing steps for roughening, masking, spraying and sealing which maybe unnecessary. The process limits certain physical characteristics of the coating to less than desirable bond strength, corrosion resistance and machineability.
Although flame sprayed epoxy coatings have not been used as dielectric coatings, they have been deployed for corrosion resistance and cosmetic applications. For such other applications, epoxy powders are usually applied by traditional powder coating processes such as electrostatic spraying followed by curing in an oven for a given time to fully activate and cure the epoxy. The art of applying epoxy by flame spray techniques has depended on the use of separate supplies of epoxide resin and hardener and has limited the type of resins that can be used because in-flight heating may not be effective to activate and cure the epoxy properly. Thus, the prior art has usually turned to plasma guns for depositing epoxy because they tend to increase or promote activation. Flame spraying has never been used to deposit epoxy coatings for electrical isolation, and when epoxy has been thermally sprayed, the powder material has not been cured in-situ as a direct result of thermal spraying.